Interproton distances commonly used in solution structure refinements of biological macromolecules are measured by the nuclear Overhauser effect (nOe), typically observable only for interproton distances <5 . The lack of longer-range distance constraints (>>5) does not normally affect the solution structures of proteins since folding brings distant primary sequence residues within 5 of each other. In duplex DNAs and some RNAs, however, the secondary structure is primarily one-dimensional with protons on each residue exhibiting nOe's only to protons on adjacent residues in primary sequence or within base-pairs. The lack of tertiary structure, coupled with the inability to determine long-range distance constraints, leaves the solution structures of many duplex oligonucleotides underdetermined. To address this concern, we have site-specifically modified a duplex undecamer (dCTCTCGGTCTC~GAGACCGAGAG) with cis-[Pt(NH3)(4AT)Cl2] (where 4AT=4-aminoTEMPO). This platinum compound is a paramagnetic analog of the potent anticancer agent cis-[Pt(NH3)2Cl2], cisplatin. Incorporation of a stable unpaired electron into an otherwise diamagnetic system induces distance-dependent relaxation of each proton by the localized unpaired electron spin density. Our work focuses on the analysis of this relaxation to determine long-range (~10-20) electron-proton distances in this paramagnetic duplex. Restrained molecular dynamics refinements of this DNA structure constrained with these long-range distances in addition to interproton (nOe) distances are in progress to determine the distortion of the dna duplex which is induced upon platinum-binding.